In Fiber Channel (FC) protocol, FEC (Forward Error Correction) core is required to perform codeword synchronization to find the frame boundary on an FC lane. The process of synchronization is important and significantly impacts the performance of FC devices.
Based on the definition of standard FC-FS-4 rev0.5, there are two kinds of codeword synchronization: “256B/257B Transmission Word synchronization” and “RS-FEC rapid codeword synchronization process”. 256B/257B Transmission Word synchronization is used to perform synchronization in FC normal operation mode. “RS-FEC rapid codeword synchronization process”, on the other hand, is used to perform rapid synchronization in FC LPI (Low Power Idle) mode.
Normal synchronization usually takes a large amount of time, the worst case goes up to 1 millisecond (ms) in 32G lane speed. For example, as defined in FC-FS-4 rev0.5, normal codeword synchronization uses the syndrome to determine whether valid codeword is found, and the candidate positions of codeword boundary are 5280. Since only one syndrome can be calculated for 5280 bit codewords every time, it is very difficult to find the codeword boundary in parallel by using the suggested method found in FC-FS-4. Rapid synchronization greatly reduces the time to find codeword boundary, but it only can be used in the LPI mode, and needs to turn down the scrambler in the Physical Coding Sublayer (PCS) core, which would impact the DC balance. Also, the valid detection window is too small to bring up risk in the synchronization process.